In a typical prior art magnetic disk recording system 10 as illustrated in FIG. 1 a slider 13 containing magnetic transducers for reading 14 and writing (not shown) magnetic transitions is urged toward the rotating disk 16 by a suspension (not shown). The magnetic sensor 14 is flanked by shields S1 and S2. As the disk rotates an air-bearing develops under the slider and causes it to fly. The distance between the slider and the disk surface is the clearance. The disk 16 typically includes a thin film overcoat 17 and a set of thin films 18 which include one or more ferromagnetic layers in which information is recorded. A disk drive can contain multiple disks and multiple sliders.
A lower slider fly-height is required for higher areal densities. Fly-heights of less than 7 nm are needed. Accordingly, it is becoming increasingly difficult to measure the actual fly-height of sliders. The most common technique using white light interferometry works adequately for a fly-height above 10 nm, but it looses accuracy below 10 nm. In addition, it is very sensitive to the optical constants of the slider overcoat. Another technique uses small solid bumps of known height on the disk and measures the acoustic emissions as the slider passes over or hits the bumps. Since it has proved impractical to make bumps below 5 nm in height, this particular technique looses accuracy for very small fly-heights.
In published U.S. patent application 20030067698 (Apr. 10, 2003) by Dakroub, et al. a method is described for measuring the fly-height of a slider using a fixed source write signal applied to an inductive element of the write head, generating a magnetic field with a stray magnetic field portion. A magnetoresistive element of the read head coupled with the magnetic field produces an initial field strength signal. As the disk rotates, the slider flies above the disk decreasing the density of the stray magnetic field portion, thereby, decreasing the amplitude of the field strength signal. The decreasing amplitude of the field strength signal is calibrated to a predetermined operating fly-height profile to correlate fly-height of the slider to the decreasing amplitude of the field strength field.
A method for measuring the fly-height of a slider using a thermal source and a thermal detector is described in published U.S. patent application 20030058559 (Mar. 27, 2003) by Brand, et al.
A laser diode generates light which is reflected off of the disk to a semiconductor laser light detector in the system described in published U.S. patent application 20030007279 (Jan. 9, 2003) by Johnson, et al.
In published U.S. patent application 20020176185 (Nov. 28, 2002) Fayeulle, et al., a method for measuring the fly-height of a slider using a disk with radial grooves in the surface is described. As the disk rotates, the radial grooves pass under the slider and induce a perturbation in the signal. By measuring the perturbations in the signal caused by the grooves in the disk surface, and by processing the measurement signal, a vertical spacing signal proportional to the vertical spacing between the disk and the head can be obtained.
In published U.S. patent application 20020071196 (Jun. 13, 2002) by Chapin, et al., a fly-height measurement is made measuring the frequency of the read/write head in response to perturbation of the gap between the read/write head and the data storage surface to invoke a dynamic response in the read/write head. A numerical model describing a functional interrelationship between the fly-height and the read/write head frequency is used. The method is described in a bench-top embodiment which uses a laser doppler vibrometer for measuring the dynamic response. The laser doppler vibrometer can be equipped with a quad-cell supplement on the laser apparatus in order to determine the pitch and roll of the slider as well. A disk drive embodiment is also described wherein the drive is able to make a self-measurement of the fly-height and take failure prevention actions is the results indicate an imminent failure. The perturbations described include a film of varying thickness on the disk, such as a Langmuir-Blodgett fluid film. It is suggested that the speed of the rotating disk can be modulated to effect a desired perturbation. Also, the inherent microwaviness of a disc can be used if sufficient to invoke resonance in the slider. In one embodiment an acoustic emissions sensor is used to determine the dynamic response of the slider to a known perturbation.
In published U.S. patent application 20020097517 (Jul. 25, 2002) by Bonin, et al., a slider is described with a field emission sensor that senses fly-height. The sensor has an electrode tip disposed on the slider. The electrode tip faces a media surface across a gap. The sensor conducts a tunneling current through the gap and provides an output representing the length of the gap.
Published U.S. patent application 20020001151 Jan. 3, 2002 by, J. H. Lake describes a system for measuring fly-height of a slider over a rotatable magnetic disk. The system determines the fly-height of the slider based on the measured pulse width and amplitude of the read back signal.
In published U.S. patent application 20010035960 (Nov. 1, 2001) by M. M. Johnston a method is presented of fly-height testing using a light emitting diode which generates a light that is reflected off the medium and the slider. The reflected light is directed to detectors capable of generating an electrical signal based on the amplitude of at least one wavelength of light in the reflected light. A distance calculator then determines the distance from the slider to the medium based on the electrical signal.
Interferometers including single beam interferometers which measure phase change of the returning light are commercially available. One such device is manufactured under the brand name Polytec and is called a “Digital LDV.” In the following the term interferometer is used in the description of some embodiments and refers to a single beam interferometer. Other suitable types of interferometers can be used and preferably should have the capability of measuring low frequency changes.